Electron discharge device



0t.3,1950 E. E. SHED-ON' 2,524,606 I ELECTRON DISCHARGE DEVICE Filed Feb. 2, 1948 INVENTOR. Edward'E/ic Shelton Hita/Hey Patented Oct. 3, 195() 'ram ottici:

ELECTRON DISCHARGE DEVICE Edward Eric Shelton,

London, England, assigner to A. C. @essor Limited, London, England, a

British company.

. Application February 2, 1948, Serial N e, 5,847 En Great Britain May 26, 1939 secties i, russe `raw 69e, august 8, 194e latent-expires May 26, 1959 This invention relates tocathode ray tubes -of the kind in which the beam is deflected successively in two mutually perpendicular directions, and in which at least the second of these deflections (herein called the X deflection) is produced electrostatically. In such cases it is customary, although not essential, for the firstdeflection (herein called the Y deflection) also to be produced electrostatically.

The principal object of the invention is to avoid deflection amplitude distortions (namely, trapezium, pin-cushion and barrel distortions) of the raster when the deflection voltages are ap plied asymmetrically and when large angles of` deliection are produced.

According to the principal feature of the nvention, there is provided a double electrostatic deflection cathode ray tube having an interplate screen and wherein both of the X deiiection plates have their entrance and emergent edges curved concavely, and have at least part of their surfaces curved conveidy toward each other so thattheir central parts are closer together than their sides, whereby deflection amplitude distortions are substantially eliminated when the tube is operated with total angles of deiiection up to at least about thirty degrees, even though the potentials of one X deection plate-andlone Y deflection plate be held constant and the deflection voltages be applied wholly to the other deflection plates.

By the term interplate screenl in this speci-Av v cation is meant an electrode placed across the path of the beam, between the region ofYdeectio-n and the region of X deflection, curved spherically about substantially the apparent `cen-- tre of Y deflection, `or-cylindrically about an axis passing substantially through that centre and perpendicular to the plan-e of Y deflection, and slotted so as to allow passage of the beam at all required angles of:Y deflection. This inter plate screen is maintained at constant pctential when the tube is in operation, usually the` potential of the nal anode of the electron gun.

In the modiliedform of the invention wherein the Y deflection is produced electromagnetb cally insteadV of electrostatically; this screen is still required, although the term interplate screen becomes inappropriate.

The effect of the interplate screen is presumed to be the shaping of the equifpotential-lines of `the eld between theentrance'edges of the plates so that they are crossed substantially atcright angles by the beam under all degrees of Y deflection. This prevents the refraction ofthe elec- 7 claims. (c1. 25o-15s) tron beam towards the axis when entering the space between the X plates and trapezium distortion due to this cause is, therefore, eliminated. The curvature of the entrance edges of the X" plates is for the same purpose.

The emergent edges of the plates are curved concavely for the similar .purpose of preventing or compensating for refraction of the beam in the direction of Y deection when emerging from the space between the X plates.

When the entrance and emergent edges of the X plates are both curved concavely, the length of each plate in its middle is considerably less than its length at its sides. Compensation for the resulting variation with Y deflection of X deflection sensitivity, known as pin-cushion distortion, is effected by the curving of at least part of the surfaces of the X plates convexly toward each other so that their central parts are closer together thanvtheir sides.

According to a preferred feature of the invention an additional electrode is provided beyond each X deflection plate, or at least beyond the working X plate (i. e. that X deection plate which is arranged for its potential to be varied during operation while the potential of the other is kept constant) said additional electrode being situated outside the extreme deflected path of the beam and being arranged to be operated at a constant potential, whereby the collection by the said X plate of secondary electrons returning from the screen is substantially prevented. A further effect of this additional electrode is to reduce the defocussing of the beam which tends to occur when the beam is deflected to pass close to the working X plate. The conductiveA coating on the wall of the envelope, or the shield surrounding the deflector plates, iprovided for the purpose of collecting secondary electrons returning from the screen is preferably maintained at the same constant'potential as this additional electrode, which may be the highest potential attained by the working X plate during operation.

The present invention will be more fully under'- stoc-d by reference to the following detailed description, which is accompanied by the drawings in which:

Fig. 1 is an elevational view of the deflecting electrode construction of a cathode ray tube embodying the principles of the present invention, while Fig. 2 is a horizontal sectional View of the electrode structure shown in Fig. 1J taken along the linelZ--Z of Fig. l,

Fig. 3 is an exploded view in perspective of one pair of deflection plates of the deilecting structure shown in Figs. 1 and 2, while Fig. 4 is a diagrammatic representation of a cathode ray tube embodying the principles of the present invention in which there is generally indicated the arrangement of the various electrodes within the tube and the potentials applied thereto.

Referring now to Figs. 1 and 2 of the drawing, reference character I indicates the final anode of an electron gun which projects an electron beam generated at a cathode (not sho-wn) along the axis of the electrode structure. The beam, after passing through the central aperture 2 inanode I, passes between the Y deiiecting plates 3 and 4 and through the slot 5a in interplate screen 5. Its path continues between the X deflecting plates 6 and 1 and between the secondary electron collector electrodes 8 and 9 and is directed toward a screen at the end of envelope 30 (Fig. 4). It will be noted that each of the Y deflecting plates 3 and 4 are so bent intermediate their ends so as to form a gradually increasing space between them toward the exit. After emerging from the region between the Y deflecting plates 3 and 4, the electron beam passes through aperture 5a in the inter-plate screen 5. Inter-plate screen 5 is an electrode placed across the path of the beam and curved symmetrically about an axis passing substantially through the apparent center of Y deflection and slotted so as to allow passage of the beam through aperture 5a at all required angles of Y deection.

As indicated by the -imark in Fig. 4, the interplate screen 5 is maintained at a constant potential when the tube is in operation, usually that of the final anode of the electron gun'. The effect of the inter-plate screen 5 is presumed to be a shaping of the equi-potential lines of the eld between the entrance edges of the X plates 6 and 1, so that they are crossed substantially at right angles by the beam under all degrees of Y deflection. This prevents the refraction of the electron beam towards the axis when entering the space between the X plates 6 and 1. Trapezium distortion due to this cause is therefore eliminated. The final deflection plates 6 and 1 are identical with each other. The entrance edges of each are curved concavely as indicated at 6a and 1a and the emergent edges are also curved concavely as indicated at 6b and 1b. The curvature of the emergent edges 6b and 1b is not constant. There is a straightportion in the middle of each, which was found necessary to avoid over-correction of the trapezium distortion for angles of deflection smaller than about Each of the plates 6 and 1 is made up of two plane sections I1 and I8 and a curved middle section I9. Plane sections I1 are parallel one to the other while the curved sections and exit sections I8 form a divergent anglealong the undeflected path of the beam. The curvature of the middle section I9 is such that all sections thereof cut by planes parallel to the plane of Fig. 2 are rectilinear. The curvature is convex inwardly so that the central part I9 of plate 6 is closer to the central part I9 lof plate 1, than are the upper and lower edge portions of plate 6, where lips are provided, with regard to the corresponding sides of plate 1. Plates 6 and 1 are reinforced by the turned-over lip portions 20 at their upper and lower edges.

The various curvatures of plates 6 and 1 and the relationship of one plate with respect to the other is shown more clearly in Fig. 3 wherein a perspective view of these two plates is shown, the spacing between the plates being somewhat increased so that one plate does not lie over the other in the figure. It will be appreciated that for any particular tube, the necessary curvatures for the inter-plate screen 5 and for the entrance and emergent edges of the X deflecting plates 6 and 1 Imay require empirical determination, but the electrode system shown in the accompanying drawing has been found to eliminate deflection amplitude distortion satisfactorily for total angles of deflection up to that is 15 to each side of the central axis occupied by the undeflected beam.

In Fig. 4 is shown a schematic representation of a cathode ray tube having an evacuated envelope 30 having a funnel-shaped conguration. The fluorescent screen (not shown) occupies the large end wall of the envelope while the electron gun structure and deflecting electrode system occupies the neck of the envelope. One of the Y deflecting plates 3 may have a saw tooth deflecting wave of low frequency applied to it as indicated by the saw tooth figure at the end of the connection lead connected to plate 3, while the other, 4, is held at a constant positive potential as indicated by the -I mark. The interplate screen 5 may be directly connected to plate 4 or it may be separately energized from `a fixed potential power equal to that of plate c as indicated in the figure. Similarly, plate 1 is held at a fixed potential while plate 6 has a high fref quency saw tooth wave applied thereto.

When the driving circuit connected to the working X plate in Fig. 4 shows a high impedance, there is a tendency to deilection amplitude distortion which varies with the intensity of the electron beam. This is believed to be due to the collection, by the working X plate, of secondary electrons returning from the screen on the end of the tube. This may be substantially prevented by the lprovision of an additional electrode such as those shown at and 9, beyond the working X plate and operated at a constant potential preferably about as high as the maximum potential attained by the working X plate during operation.

The envelope 30 of the cathode ray tube is preferably provided with a conductive coating 3! on the inside of the envelope which is maintained at the same potential and may be directly connected to the additional electrodes 8 and 9 as indicated in Fig. 4. The tube illustrated in the drawing is entirely symmetrical, and an additional electrode 8 or 9 is provided for each X deflecting plate 6 or 1. It is, therefore, immaterial which of the X plates 6 and 1 is held at fixed potential, and which is used as the working plate in Fig. 4. Merely for the sake of illustration, plate 1 is indicated as being held at a xed potential.

The present invention will principally be applied to cathode ray tubes having fluorescent screens, but it is evident that the precise nature of the screen is irrelevant to the invention, and the invention may be used in all cases where asymmetric distortion voltages are employed, and wide angles of deflection are required. Also, the invention is applicable to cathode ray tubes of the gas lled type, as well as to those in which the focusing of the beam is mainly affected by electro-static elds. As, however, the beam current of gas lled tubes is not modulated during operation, the secondary electron collecter electrodes 8 and 9 do not serve the same total purpose, though they still act to increase the apparent plate to plate impedance presented to the driving circuit.

I claim:

1. A cathode ray tube of the kind in which an electron beam from a cathode is deflected suc cessively in two mutually perpendicular directions, the rst of these deflections being termed the Y deflection and the second of these deflections being termed the X deilection and is produced electrostatically by a pair of deflecting plates, said tube comprising Y-deflecting means for producing the said Y deflection, and an interplate screen interposed between the said Y- deecting means and the said pair of deflecting plates, said interplate screen being in the form of an electrode curved concavely towards said sY-deflecting means and being slotted to allow passage of the beam over a range of Y deections, both of said X deflection plates having entrant and emergent edges curved concavely, and said X deflecting plates having at least part of their facing surfaces curved convexly towards each other so that their central parts are closer together than their side portions.

2. A cathode ray tube according to claim 1, wherein an additional electrode is provided beyond that X deflection plate which is arranged for its potential to be varied during operation, said additional electrode being arranged outside the extreme deflected path of the beam and being arranged to be operated at a constant potential whereby the collection by the said X plate of secondary electrons returning from the screen is substantially prevented.

3. A cathode ray tube according to claim 1, wherein said X-plates are symmetrically constructed for operation with either of the X deflection plates held at constant potential and with the deflection voltages applied to the other of such X deflection plates, and including an additional electrode is provided beyond the X deflection plate which is arranged to receive a variable potential during operation, said additional electrode being arranged outside the eXtreme deflected path of the electron beam and being arranged to be operated at a constant potential whereby the collection by the said X plate of secondary electrons returning from the screen is substantially prevented. y

4. A cathode ray tube of the kind in which an electron beam from a cathode is deected successively in two mutually perpendicular directions, the second of these deections being termed the X deflection and being produced electrostatically by a pair of deflecting plates, both of the X deection plates having entrance and emergent edges curved concavely, said plates having at least part of their facing surfaces curved conveXly toward each other so that their central parts are closed together than their sides.

5. A cathode ray tube according to claim 4 wherein the potential of one of said X deection platesis held constant and the other of said X deflection plates has deflection voltages applied thereto and wherein an additional electrode is provided beyond the latter of said X deflection plates, said additional electrode being operated at a constant potential whereby the collection of secondary electrons by said latter plate is substantially prevented. Y

6. A cathode ray tube, having within an evacuated envelope a source for a beam of electrons, a first pair of opposing deilection plates and a second pair of opposing deflection plates arranged in planes perpendicular to said rst pair of plates, said second pair of plates havingentrance and emergent edges curved concavely and at least part of their facing surfaces curved convexly toward each other, so that their central parts are closer together than the side parts whereby deflection amplitude distortions are substantially eliminated, the potential of one of said rst pair of deflection plates and one of said second pair of deflection plates being held constant and the other of each of said pairs of plates having deilection voltages applied thereto.

7. A cathode ray tube according to claim 6, wherein an additional electrode is provided beyond that one of said second pair of deflection plates which is arranged for its potential to be varied during operation and outside the path of said beam when grazing the said one of said second pair of deflection plates, said additional electrode being operated at a constant potential whereby the collection by said plate of secondary electrons is substantially prevented.

EDWARD ERIC SHELTON.

REFERENCES CITED The following references are of record le of this patent:

UNITED STATES PATENTS in the 

